The invention described herein relates generally to short wavelength lasers, and more particularly to electron collisionally-excited EUV (extreme ultraviolet) or soft X-ray lasers.
During a period extending back from the present time for more than two decades, an intense search has been underway for ways of achieving, in the laboratory, laser emission at EUV or soft X-ray wavelengths. Coherent beams of photons produced by these lasers will have many beneficial scientific purposes, such as submicroscopic imaging, holography and spectroscopy. Although population inversions in plasmas, such as may be expected to lead to EUV or soft X-ray lasing, have been reported, no conventional-laser-driven EUV or soft X-ray laser has yet been reported.
One proposed generic conventional-laser-driven short wavelength laser scheme that is currently being actively studied is the electron collisionally-excited approach that is dissussed by Zherikhin et al in Sov. J. Quant. Electron. 6, 82 (1976). Zherikhin et al point out that electron collisions may establish a population inversion in ions with a ground electronic configuration 1s.sup.2 2s.sup.2 2p.sup.m as a result of 2p.sup.m-1 3s-2p.sup.m-1 3p transitions. The upper state 2p.sup.m-1 3p decays radiatively most effectively to the 2p.sup.m-1 3s state, which, in its turn, decays radiatively and very rapidly to the ground state, whereas the radiative decay of the 2p.sup.m-1 3p state to the ground state is forbidden. Both of the inverted laser state levels are populated from the ground state as the result of electron impact. Zherikhin et al state that neon-like ions are most suitable for this scheme because, due to a jump of the ionization potential between ions with outer L shell electrons to ions with outer M shell electrons, and even though fast ion recombination may occur in an expanding plasma, among ions with the 2p.sup.m configuration neon-like ions with the 2p.sup.6 configuration are the longest-lived. Neon-like ions are atoms having an atomic number greater than ten that are stripped of all but ten of their usual compliment of electrons. Zherikhin et al calculate appreciable gains for plasmas composed of neon-like ions of elements in the atomic number range from 16 to 25 heated by two-stage laser pumping wherein the electron component of a previously generated laser plasma filament is rapidly heated by an ultrashort pulse of high-power laser radiation traveling along the plasma filament. Because of theoretical difficulties, Zherikhin et al state that it is not clear whether the method can be extended to high atomic number plasma systems.
Vinogradov et al in Sov. J. Quantum Electron. 7, 32 (1977), in considering tne electron collisionally-excited approach to conventional-laser-driven short wavelength lasing under discussion, theoretically conclude that 3p-3s transitions can be inverted in optically thin steady-state plasmas comprising neon-like ions carrying a charge between 7 and 15. Specific numerical results are given for the Ca XI ion. Calcium has the atomic number 20. An important finding of Vinogradov et al is that two-stage laser pumping is not an absolute theoretical requirement of this electron collisionally-excited method.
As currently understood, the electron collisionally-excited single pass EUV or soft X-ray laser scheme involves using a driving conventional laser to produce a mid- to high-density plasma of neon-like ions. Strong monopole electron collisional excitation from the ground state of the neon-like ions fills 3p states. This inverts 3p to 3s transitions because the lower energy 3s states radiatively decay very rapidly. Although the physics of the scheme is complex, it is nevertheless believed that strong 3p excitations may occur for neon-like ions produced from elements having an atomic number near 36 in systems driven by 0.53 micron wavelength laser light at an intensity of about 10.sup.14 watts/cm.sup.2. The gain, usually stated in terms of reciprocal centimeters, of a transition produced by this scheme is believed to be a function of the parameters of the driving conventional-laser pulse, the atomic number of the element comprising the plasma, the free electron density of the plasma, the electron temperature of the plasma, and, because of the potential of radiation trapping, the dimensions of the plasma. Two-stage laser pumping is not required.
An attempt was made to experimentally test the electron collisionally-excited single pass EUV or soft X-ray laser scheme at the NOVETTE laser facility of the Lawrence Livermore National Laboratory. The experimental arrangement is schematically shown in FIG. 1, prior art, to which reference is now made. Laser pulse 10, comprised of a 200 picosecond full width at half maximum amplitude, 0.53 micron wavelength, cylindrically focused light pulse having an average intensity of approximately 10.sup.14 watts/cm.sup.2, was directed onto selenium panel 12, which was approximately 1,000 Angstroms thick. Selenium panel 12 was coated on parylene substrate 14, which was approximately 0.5 microns thick. Parylene substrate 14 was supported within aluminum trough 16. Laser pulse 10 caused a plasma, formed from blown-off selenium atoms, to come into existence adjacent to selenium panel 12. According to calculations performed on the Lawrence Livermore National Laboratory LASNEX computer code, and other computer codes, conditions within the plasma should have been such as to produce lasing emission at approximately 68 eV from neon-like selenium atoms by the electron collisionally-excited mechanism. Particularly, according to the calculations, the plasma was expected to have, over an extended period of time, the electron density and gain, as functions of the distance from the surface of selenium panel 12, shown in FIG. 2, prior art, and in FIG. 3, prior art, respectively. During the lasing time, the plasma was expected to have an approximately constant electron temperature of about 800 to 1500 eV. An X-ray detector which was carefully adjusted to measure radiation in the 58 to 78 eV energy range that was within an approximately 0.005 radian acceptance angle, monitored axial radiation emission, in a direction through the plasma and parallel to the surface of selenium panel 12, from the high gain portion of the plasma that was confined within approximately 30 microns of the surface of selenium panel 12. Laser amplification was not detected. Since the plasma was optically thin on the 2p to 3s line coupling the ground state to the lower laser state, radiation trapping is an unlikely cause of this null result.
Thus, even though the theory underlying the generic electron collisionally-excited single pass conventional-laser-driven short wavelength laser mechanism is believed to be valid, it is not known how to construct an operational short wavelength laser of this type.